When I varnished my Chesapeake a few weeks ago, I supported the boat on extensions that went into the fore and aft hatches.  This allowed for me to flip the boat right away.  I'd say to give it at least a week to allow things to cure up before putting the fresh varnish on straps.  Then again, I use epifanes, which is a softer varnish and takes longer to cure.  If you can support it without putting fresh varnish on another surface, all the better.

 ~Chris

  I am getting more anxious about doing the varnishing.  My first time doing this -- now I need to build a sling?  But wait a week before applying it to straps?  Dang.  So then it is better to do the hull of my boat first, letting the boat rest on the coaming, then fliip it after a week.  That way I don't need a sling at all. 

Dawn/Dave,

Another option (seen on this forum, but I can't recall where/by whom) is to hang the kayak using a 2x4 inside the hatches or cockpit.  In other words, the board runs under each edge of the opening and you hang the boat by two straps, fore and aft.  I did that while painting my hull and was able to paint everything (stripe on deck and solid hull) at one time. 

Just a thought - but it worked great for me,

Larry